Waste liquid or aqueous media containing toxic materials has presented an acute disposal problem. However, in accordance with the inventions described and claimed in U.S. Pat. Nos. 3,926,754; 4,036,726 and 4,123,339 assigned to the assignee of this invention, heavy metals such as hexavalent chromium ions were removed from cooling tower wastewater rapidly and efficiently, electrochemically. The disclosures of these patents are hereby incorporated by reference.
In the above patents a process and apparatus were described wherein wastewater containing hexavalent chromium ions is caused to flow between a plurality of electrodes. It was discovered that when the anode has a surface or a portion of the surface of iron, an iron alloy or an insoluble iron compound an iron compound such as iron hydroxide will be produced electrochemically. In turn an insoluble trivalent chromium compound, preferably as the hydroxide, will be produced which will complex with or otherwise physically or chemically combine with the insoluble iron compound to thereby permit removal from solution. Whereas it was previously considered necessary to reduce hexavalent chromium to trivalent chromium in acidic solution, it was discovered that the iron compound or complex formed will reduce the hexavalent chromium and coprecipitate therewith in a solution having a pH of about 4 to about 11. Accordingly, the invention described and claimed in said patents produces an insoluble iron-chromium precipitate without pH adjustment to thereby rapidly and efficiently remove hexavalent chromium from solution. The precipitate is then removed from aqueous media utilizing conventional techniques such as a clarifier, settling pond or the like and the aqueous media thereby clarified is suitable for disposal.
In this process, hexavalent chromium undergoes cathodic reduction to form trivalent chromium as insoluble chromichydroxide which complexes with iron which enters solution at the anode. The products are not susceptible to further electrolytic oxidation at the anode, back to hexavalent chromium, apparently due to the difference in ionization potential, at least in part, because the production of the hydroxide ion at the cathode occurs at a much lower potential than other electrode reactions. Thus, because of the non amphoteric state of the iron complex, the reaction continues until the undesirable contaminating ions are completely or substantially completely removed from solution in the aqueous media.